Make private inline accessors final

Fixes #15323

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/AccessProxies.scala

@@ -76,17 +76,18 @@ abstract class AccessProxies {
     }
 
     /** A fresh accessor symbol */
-    private def newAccessorSymbol(owner: Symbol, name: TermName, info: Type, span: Span)(using Context): TermSymbol = {
-      val sym = newSymbol(owner, name, Synthetic | Method, info, coord = span).entered
-      if (sym.allOverriddenSymbols.exists(!_.is(Deferred))) sym.setFlag(Override)
+    private def newAccessorSymbol(owner: Symbol, name: TermName, info: Type, accessed: Symbol)(using Context): TermSymbol = {
+      val sym = newSymbol(owner, name, Synthetic | Method, info, coord = accessed.span).entered
+      if accessed.is(Private) then sym.setFlag(Final)
+      else if sym.allOverriddenSymbols.exists(!_.is(Deferred)) then sym.setFlag(Override)
       sym
     }
 
     /** An accessor symbol, create a fresh one unless one exists already */
     protected def accessorSymbol(owner: Symbol, accessorName: TermName, accessorInfo: Type, accessed: Symbol)(using Context): Symbol = {
       def refersToAccessed(sym: Symbol) = accessedBy.get(sym).contains(accessed)
       owner.info.decl(accessorName).suchThat(refersToAccessed).symbol.orElse {
-        val acc = newAccessorSymbol(owner, accessorName, accessorInfo, accessed.span)
+        val acc = newAccessorSymbol(owner, accessorName, accessorInfo, accessed)
         accessedBy(acc) = accessed
         acc
       }

tests/neg/i15323.scala

@@ -0,0 +1,11 @@
+class A:
+  private var x = 0
+  inline def f1(): Int =
+    x += 1;
+    x
+
+class B extends A:
+  private var x = 0 // error
+  inline def f2(): Int =
+    x += 1
+    x